This invention is related to the production of aromatic dicarboxylic acids. More particularly this invention is related to a process for selectively carboxylating an aromatic mono-acid to form primarily an aromatic diacid. One embodiment of the invention is the selective carboxylation of naphthoic acid to form primarily 2,3-naphthalene dicarboxylic acid (2,3-NDA). A second part of the invention is the incorporation of the selective carboxylation into a two-stage process for producing greatly increased yields of aromatic diacid. The invention makes greater use of aromatic rings and obtains a surprisingly high yield of an aromatic dicarboxylic acid, such as, for example, 2,6-naphthalene dicarboxylic acid (2,6-NDA).
Aromatic dicarboxylic acids are highly useful organic compounds. They are often used as monomers for the preparation of polymeric materials. For example, terephthalic acid is used to prepare polyethylene terephthalate, a widely used polyester material and the naphthalene dicarboxylic acids, i. e. 2,6-naphthalene dicarboxylic acid, is a particularly useful aromatic carboxylic acid because it can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate), PEN. Fibers and films manufactured from PEN display improved strength and superior thermal properties compared with other polyester materials such as polyethylene terephthalate. High strength fibers made from PEN can be used to make tire cords, and films made from PEN are advantageously used to manufacture magnetic recording tape and components for electronic applications. It is desirable to use as pure as possible forms of these dicarboxylic acids in the various applications. It is also desirable to obtain as high a yield as possible of the aromatic dicarboxylic acids.
It is known in the art to prepare aromatic dicarboxylic acids by primarily two methods. One is the liquid phase, metal catalyzed oxidation of an alkyl or acyl substituted aromatic compound. This method is described, for example, in U.S. Pat. Nos. 2,833,816; 3,856,855; 3,870,754; 4,933,491; and 4,950,786. This method has drawbacks. The primary disadvantage of the method that involves direct oxidation to 2,6 NDA, is that impurities are trapped in the 2,6 NDA oxidation product which forms upon oxidation as a solid in the oxidation solvent. In order to remove these impurities to a sufficiently low level acceptable for polymerization, the 2,6 NDA product must be purified via multiple steps. These steps typically involve esterification, so that the resulting end product is 2,6-naphthalene dicarboxylate, an ester, rather than the preferred 2,6 napthalene dicarboxylic acid.
Alternatively, naphthalene monocarboxylic acid and naphthalene dicarboxylic acids other than 2,6-naphthalene dicarboxylic acid can be converted to 2,6-NDA using a disproportionation reaction in the case of the monocarboxylic acids or a rearrangement reaction in the case of other naphthalene dicarboxylic acids. Henkel and Cie first patented a reaction of naphthoic acid salts to 2,6 NDA in the late 1950s. (See U.S. Pat. Nos. 2,823,231 and 2,849,482). In these references, it can be observed that excess base was neutralized out of the feed with HCl with the objective of having precisely a 1:1 ratio of K:carboxyl. These references demonstrate the disproportionation of benzene to terephthalic acid+benzene. Isomerization of a diacid such as phthalic to terephthalic was demonstrated, as well. It can be observed that the best yield of diacid in this work was about 65%.
It is known in the art that in normal Henkel disproportionation reactions, a significant yield loss occurs during the reaction. This loss, even in the best of circumstances, is usually 3% or more of the weight of the naphthalene dicarboxylic acids (NDAs) theoretically expected to be produced. This loss arises from a mixture of causes, such as coupling of aromatic radicals to form binaphthyls and higher condensed species, decarboxylation of naphthoic acids to naphthalene, and other undesired reactions.
In the absence of charging other carboxylic acid salts (e.g. tricarboxylic benzene acids, or potassium formates, and the like) there is no precedent for obtaining a yield of NDA which exceeds the theoretical yield given by the equation for the Henkel II reaction:
2(potassium naphthoate)xe2x86x921 napthalene+1 naphthalene dicarboxylic acid,
where the naphthalene dicarboxylic acid is a mixture of isomers, usually mostly 2,6-naphthalene dicarboxylic acid.
A perplexing question has been how one could more fully use all of the rings present in a feed of aromatic monoacids without the need for alkylation or subsequent oxidation. For example, there has not been a method available in the art to fully use all of the naphthalene rings present in a feed of naphthoic acid.
There does not appear to be any work in the art relating to the possibility of selective carboxylation of monoacids to aromatic diacids using inorganic salts. One Japanese reference claims carboxylation in the presence of oxalates, another organic salt, however only very low molar conversion was demonstrated, with only about 2% of the carboxyl groups present in the oxalate being transferred. (cite unavailable)
There is a great demand for dicarboxylic acids in the production of polymers, yet it has been difficult to produce dicarboxylic acids of good purity and in high yields. It would be a great advance in the art if it were possible to significantly increase the yield of dicarboxylic acids in a disproportionation/isomerization type reaction.
If there were a method available for direct carboxylation of an aromatic monoacid it would provide a significant advance in the art. It would be particularly valuable if there were a method for producing the much sought after 2,6-napthalene dicarboxylic acid in significantly greater yields by direct carboxylation of a feed which is simple to purify and oxidize, such as napthoic acid.
In the present invention it has been discovered that by operating in an unusual regime of high base and lower temperature, it is possible to produce a significantly higher ratio of NDA to naphthalene than the theoretical ratio of 1.0.
In accordance with the foregoing, the present invention comprises a method of directly carboxylating an aromatic monoacid to an aromatic diacid, which comprises:
Reacting said aromatic monoacid with excess base in the presence of a catalyst comprising a metal oxide, particularly an oxide of Group IIB, at a temperature of from about 350xc2x0 to 500xc2x0.
A second embodiment of the present invention also comprises substantially increasing the yield per pass in a disproportionation/isomerization reaction by a two-stage process comprising:
Heating overbased naphthoic acid salt at a temperature up to about 420xc2x0 C. for a relatively short period of time to form 2,3-NDA by carboxylation, followed by a heating the product for a relatively longer period of time at a higher temperature, say above 420xc2x0 C., to isomerize the mainly 2,3-NDA product of the first step to 2,6-NDA.
The invention demonstrates an increase in yield per pass in the disproportionation reaction to form naphthalene dicarboxylic acid, as well as increased throughput, and the reduced recycling of naphthalene. The present invention more fully utilizes all of the naphthalene rings present in a naphthoic acid feed to form naphthalene dicarboxylic acid, without the need for alkylation or subsequent oxidation. The examples demonstrate the direct carboxylation of 2-naphthoic acid, and mixtures of 1- and 2-naphthoic.
The present invention makes it possible to have a low capital, highly efficient disproportionation/isomerization type process to produce naphthalene dicarboxylic acid from naphthoic acid without the need for recycle of naphthalene for alkylation to naphthoic acid. The present invention could greatly simplify and make more productive any process for producing aromatic dicarboxylic acids, especially those that utilize a disproportionation/isomerization reaction.
Disproportionation reactions, such as the Henkel reaction, which are known in the art, to produce aromatic dicarboxylic acids, particularly naphthalene dicarboxylic acid, can be represented by the following: 
In this reaction, the ratio of base to acid is 1:1. In fact, in early work, HCl was employed in the reaction to neutralize all excess base out of the feed. The best yield of 2,6-NDA demonstrated in early work was about 65%.
In copending U.S. Application Serial No. 60/151,577 incorporated by reference herein in the entirety, the yield of 2,6-NDA in an integrated process incorporating a disproportionation reaction has been optimized to only about a 3% yield loss.
Carboxylation
The present invention comprises the discovery of a method to selectively carboxylate naphthoic acid, or other aromatic mono-acids, to form primarily 2,3-naphthalene dicarboxylic acid (2,3-NDA) or other aromatic diacids. This reaction can be represented by: 
The process includes the use of an excess of basic carbonates, a specific temperature range, and salt feeds of particular X-ray diffraction characteristics. In addition, thermogravimetric analysis (TGA) studies of the feeds suitable for the present invention will reveal a relatively low onset temperature of non-drying weight loss on heating (see Table 2). Differential Scanning Calorimetry(DSC) studies also show such feeds to exhibit a low melting transition (50xc2x0 C. or more below the normal potassium naphthoate, KNA, melting point of 410xc2x0 C.) associated with a separate phase.
It is not certain, but it is thought the reaction of the present invention proceeds via the formation of a salt bond between the aromatic monoacid salt and a molecule of bicarbonate or carbonate salt, with the subsequent formation of an aromatic diacid disalt with the COO(xe2x88x92)M(+), (where M is the metal counter ion), groups adjacent to each other on the aromatic ring (e.g., phthalic acid from benzoic acid, 2,3-NDA from 2-NA, 3,4-BDA from 4-carboxy biphenyl, etc.), and a molecule of water and carbon dioxide (from bicarbonate) or of bicarbonate(from carbonate) containing the hydrogen atom removed from the aromatic ring.
Successful practice of the process of this invention requires sufficient mobility to allow the intermolecular carboxyl transfer to occur via the salt bridge, and sufficient stability to avoid decarboxylation. Each system requires, therefore, a specific temperature and pressure range to be effective. However, the key element of the invention is the use of excess carbonate, bicarbonate, or related base, which furnishes, ultimately from carbon dioxide gas, the carboxyl groups to be transferred to the ring.
The starting material for the present invention is an aromatic monoacid. Suitable examples include, but are not limited to benzoic, 1-naphthoic, and 2-naphthoic. Alkyl substituted aromatic monoacids will work.
The excess base is a critical element of the present invention. The optimum level of overbasing is between 0.1 and 1 moles of excess base per mole of acid, although this is probably a function of the exact formulation and conditions used. Using potassium carbonate or bicarbonate and naphthoic acid, a suitable range of overbasing is 1.05-1.8:1, moles potassium to moles of acid. Good results were obtained using 1.2-1.6:1 moles of K per mole of acid.
Suitable bases include alkali metal carbonates. Bases used to provide the excess include, but are not limited to, K2CO3, KHCO3, Rb2CO3, RbHCO3, Cs2CO3, CsHCO3, and other strongly basic carbonates or bicarbonates. The preferred base was potassium carbonate or potassium bicarbonate.
A suitable catalyst for the carboxylation is an oxide of a metal. This can include a number of metal oxides, but is preferably an oxide of a metal selected from IB, IIB, or IIIA of the Periodic Table, including, but not limited to zinc, cadmium, copper, indium, aluminum, and silver. Good results in the carboxylation of naphthoic acid were obtained using zinc oxide.
The temperature for carboxylation in the first embodiment of the present invention will be in a narrow range. Generally it will be about 50xc2x0 C. below a suitable temperature for disproportionation/isomerization of the starting material. The onset of carboxylation for naphthoic acid, for example, is typically about 380xc2x0 C., but may be observed in the range between about 375-385xc2x0 or 390xc2x0 C. As the temperature is increased, carboxylation takes place up to about 415-425xc2x0 C., typically about 420xc2x0 C. Above that temperature potassium salts isomerize and disproportionation begins to predominate.
Two-Stage Process
The second part of the invention, comprising a two-stage conversion to an aromatic diacid, such as 2,6-naphthalene dicarboxylic acid, consists of 1.2 to 2.4:1 potassium: naphthoic acid overbased material, processed through a low temperature stage (380xc2x0-425xc2x0 C.) to make 2,3-NDA, followed by a high temperature stage (ca.435xc2x0-455xc2x0 C.) to convert the 2,3-NDA to 2,6-NDA. The practical overbasing salts will be KHCO3 and K2CO3, and the exact temperature profile and conditions are critical. More base may be required, depending on the efficiency of utilization of the base in high conversion experiments.
This two-stage process can be effected, inter alia, by feeding overbased naphthoic acid salt through a heated screw device into a reactor of a higher temperature for a longer residence time, by feeding a slurry of such naphthoic acid salt into a small vessel or pipeline followed by a larger vessel at a higher temperature, passing hoppers of salt through first a lower and then a higher temperature zone, or other means which will be obvious to those skilled in the art to effect the two stage reaction process.
In the two-stage process it is helpful to form a slurry of the feed materials. Aromatic hydrocarbons are desired as liquid slurrying media. The slurrying media can suitably be any compound with sufficient thermal stability. It is not restricted to aromatic compounds, however aromatic compounds are suitable. Examples of suitable solvents include a single compound or mixture of compounds selected from a variety of aprotic polycyclic aromatic compounds, such as, for example, naphthalene, methylnaphthalene, dimethylnaphthalene, diphenyl ether, dinaphthyl ether, terphenyl, anthracene, phenanethrene, and mixtures thereof. The preferred medium is naphthalene.
It is demonstrated in the first stage that carboxylation proceeds up to 87% on converted naphthoic acid salt under the specified low temperature reaction conditions. (See Table 1, Ex. 5) In the second stage it is demonstrated that the 2,3-NDA and other isomers so formed may, by raising the temperature, be converted in at least 80% per pass and 98% overall yield to 2,6-NDA, giving an overall yield of ca. 20% per pass and about 95% overall to 2,6-NDA from naphthoic acid via the two step process of this invention. Higher conversions in the first step will increase the per pass yield of 2,6-NDA to ca. 70%, in excess of any values disclosed in the literature for a single step conversion of 2,3-NDA to 2,6-NDA, and more than 40% more than the theoretical yield of 2,6-NDA by disproportionation.
So far, 110% vs typical 97% disproportionation yields at 1.2:1 overbasing corresponds to about 65% net utilization of the excess base in bicarbonate vs. KOH systems.
The following examples will serve to illustrate specific embodiments of the invention disclosed herein. These examples are intended only as a means of illustration and should not be construed as limiting the scope of the invention in any way. Those skilled in the art will recognize many variations that may be made without departing from the spirit of the disclosed invention.
The examples will demonstrate that by operating in an unusual regime of high base and low temperature, it is possible to produce a significantly higher ratio of NDA to naphthalene (NDA/N) than the theoretical ratio of 1.000. Without these conditions, the best practical result is about 0.97 NDA per Naphthalene (0.97=NDA/N) for the converted K-2-NA salt. With these conditions, NDA/N ratios of greater than 1.5 may be obtained. The inventive process consists of using an excess of base, particularly an excess of the carbonate and bicarbonate salts formed by CO2 precipitation of 2,6 NDA in the acid form, and holding at lower temperatures than the conventional Henkel disproportionation (Henkel II) temperatures, most preferably from about 390xc2x0 C. to about 420xc2x0 C. Additionally, the most preferred embodiment involves preparation of a finely dispersed mixture of the excess base salts with the naphthoic acid salts, often indicated by low melting peaks in the DSC (differential scanning calorimetry) scans of the feed salts. Further, the reaction may be accelerated by addition of small amounts of larger alkali cation salts (e.g., Cs) which also seem to depress the melting point and improve the mixing of the carbonic and naphthoic salts. In the examples zinc salts were used to promote the reaction although other salts may be used, as is known in the literature (Cd, etc.). The zinc salts, preferred for cost and low toxicity, may be used as the oxide, carbonate, or other inorganic salt, or the organic salt of the naphthoic acid feed, conveniently formed by reaction of the naphthoic acid with zinc oxide under elevated temperature.
The experimental procedure comprises adding together naphthoic acid, water, and excess bicarbonate, mixing these starting materials, and heating to form potassium naphthoate, mixed with excess carbonate and bicarbonate. Alternatively, the naphthoic acid is ground up and added to an aqueous solution of K2CO3 to form a salt that is soluble in base. The mixture is heated to about 100xc2x0 C. for about 10 to 30 minutes. Those skilled in the art would realize that one could heat at a higher temperature by adding pressure to the water; and one could use a lower temperature if a longer reaction time is used. Typically, the mixture was reacted at about 100xc2x0 C. for about 20 minutes. In addition, particle size and rate of addition would be affected by the ratio of base to naphthoic acid. The naphthoic acid is dissolved in base and the water with solids is stripped.
The best results have been obtained with a formulation prepared from naphthoic acid, an excess of potassium bicarbonate, a specific drying regimen such that the starting materials exhibit specific x-ray diffraction characteristics, and thermogravimetric analysis characteristics characterized by a relatively low onset temperature of non-drying weight loss on heating (see Table 2). The drying regimen involves heating the solids at about 175xc2x0 C. for about 2 hours under 0.8 torr mm Hg pressure. Especially preferred feeds are ones that have mixtures of the following xe2x80x9ctwo thetaxe2x80x9d peaks (among others) in the powder X-ray diffraction patterns of the initial (gently dried, air exposed) feed salt mixture: 14.0, 28.5, 38.2, 13.6, 27.3, 32.0, and 36.7 degrees two theta. Not all the peaks need be there in all samples, but in general feeds which have these peaks (as well as others) will give good yields. These peaks correspond to lattice spacings of 6.32, 3.13, 2.35, 6.52, 3.26, 2.80, and 2.45 Angstroms in Bragg d-spacing.
Further description of the experimental procedure includes alternative ways of mixing the starting materials before the drying regimen:
1. The powdered acid can be added to the aqueous material.
2. The acid can be dissolved in hydrocarbon solvent at elevated temperature where water with base and catalyst are added and allowed to react as the water is boiled out of the mixture.
3. The acid and base can be contacted with each other at an elevated temperature in the range of about 90-100xc2x0 C.
4. The water phase or aqueous salts can be dripped into the hydrocarbon solvent.
5. Powdered naphthoic acid can be dripped into a solution of aqueous base with catalyst suspended.
In some examples a eutectic mixture was employed. A eutectic mixture provides the lowest melting point of a mixture of two or more alkali metals that is obtainable by varying the percentage of the components. Eutectic mixtures have a definite minimum melting point compared with other combinations of the same metals. For example, though the melting point of LiCO3 is 622xc2x0 C., in a eutectic mixture of alkali carbonates the melting point can be 400xc2x0 C. What is required in the present examples, where a eutectic mixture is employed, is the right mixture of alkali metal carbonates where the melting point is less than about 400-420xc2x0 C. Generally the ratio of alkali metal carbonates in the eutectic mixture is about 1:1:1, but it can vary. One eutectic mixture used as a solvent was K2CO3, Rb2CO3, Cs2CO3, and optionally including Na2CO3.
Best results have been obtained with ZnO as the catalyst for carboxylation, although a variety of metal oxides will work as catalysts.
Optimum temperatures for the carboxylation reaction are typically about 50xc2x0 C. below the optimum for a disproportionation/isomerization reaction as known in the art for a similar composition.
Though potassium bicarbonate works well, any xe2x80x9coverbasedxe2x80x9d potassium, rubidium, or cesium carbonate salt, preferably between 1.2:1 alkali metal:naphthoic acid mole ratio and 2.0:1 alkali metal to naphthoic acid ratio appears to make the reaction possible, if run carefully at the optimum temperature and conditions for the specific material. A preferred range of overbasing is between 1.2-1.6:1, although this is probably a function of the exact formulation and conditions used. More than half of the excess base present may be transferred to the rings as carboxyl groups. To fully convert the NA to 2,3-NDA, a full mole of excess base is required. In copending U.S. patent application Ser. No. 60/151,606, filed of even date and incorporated by reference herein in its entirety, it was shown that overbasing of 0.01 to 0.20 moles per mole of monoacid in disproportionation gives optimum 2,6-NDA yield under normal condition; and that application also showed excellent yields ( greater than 95w) of isomerization of the diacid disalts (including K2-2,3-NDA) in the presence of 0.1 to 0.2 moles of excess base.
Though the art would indicate water should be avoided in a reaction of this type, it has been found in the present invention that a small amount of water, say less than 1000 ppm, seems to actually facilitate the reaction. It is speculated the beneficial effects are due to the effect of the water of introducing increased mobility into the salts, specifically allowing the salt complexes more rotational freedom and also by stabilizing the charged species formed as intermediates. It is also possible a small amount of water stabilizes the original finely divided mixed crystalline low melting materials that make the best feeds. However, a significant amount of water, say, for example in excess of ca. 700-1000 ppm, interferes with the reaction by beginning to favor the decomposition of the salts (decarboxylation). A significant amount of water, 0.2% or more, becomes damaging by promoting yield loss.
In the second embodiment of the present invention, the two-stage process consists of 1.2 to 2.4:1 K:NA overbased material, processed through a low temperature stage (380xc2x0-425xc2x0 C.) to make 2,3-NDA, followed by a high temperature stage (ca.435xc2x0-455xc2x0 C.) to convert the 2,3-NDA to 2,6-NDA. The practical overbasing salts will be KHCO3 and K2CO3, and the exact temperature profile and conditions will be critical. More base may be required, depending on the efficiency of utilization of the base in high conversion experiments. So far, 110% vs typical 970% disproportionation yields at 1.2:1 overbasing corresponds to about 65% net utilization of the excess base in bicarbonate vs. KOH systems. If this efficiency is the limit, 2.0:1 overbasing would yield 165% of disproportionation and 180%(90% direct yield of NA to 2,6-NDA) would require ca. 2.3:1 overbasing in the above proposed range.)
Using the two-stage process of this invention, comprising carboxylation of naphthoic acid and subsequent disproportionation and isomerization to 2,6-NDA at high conversion per pass of 80% or more, yields of up to 110% of the theoretical yield for a xe2x80x9cHenkelxe2x80x9d disproportionation have been observed at conversions of naphthoic acid up to 90%. At lower conversions, higher excesses of naphthalene dicarboxylic acid have been observed, up to ca. 76% of the converted naphthoic acid.